1. Field of the Invention
This invention relates generally to lasers and more particularly to etalon stabilized, grating tuned, external cavity semiconductor lasers.
2. Description of the Prior Art
Future optical communications systems employing heterodyne methods will require turnable, single-frequency, narrow-linewidth, semiconductor lasers. Such laser systems will be modulated and coupled to optical fiber cables for transmission of the light energy to a distant receiver. This type of laser optical system has distinct bandwidth advantages over the systems currently in use which makes it attractive for high-bit-rate and long-haul optical communications systems.
For heterodyne communications and other coherent (single frequency) applications, such as heterodyne signal analysis, the line-width (optical bandwidth) of monolithic single frequency lasers may not be sufficiently narrow for acceptable system performance. External cavity, grating tuned, semiconductor lasers, however, have the narrow linewidth and tuning range needed for such applications.
The narrow linewidth of the external cavity laser is particularly important in optical heterodyne spectrum analysis equipment. In these systems, the linewidth of the local oscillator used in such equipment should be much narrower than that of the signal under test. In addition, the longitudinal sidemode suppression ratio of the local oscillator must be very high, in excess of 30db, since any sidemodes on the local oscillator would be indistinguishable from spectral features on the signal under test.
The design of grating tuned external cavity semiconductor lasers is well known in the art. An example of an external cavity laser design is disclosed in U.S. Pat. No. 4,942,583 issued to Paul Zorabedian et al. and assigned to Hewlett-Packard Company.
One drawback of external cavity lasers is their tendency to hop (switch) between two or more longitudinal mode frequencies (multimode) during tuning or when the bias current is increased to maximize the laser output power. The multimode operation occurs because the bandwidth of the external cavity laser is not sufficiently narrow to exclude all but one frequency of operation.
There are three general types of multimode operations. Although not always the case, the three types of multimode operations tend to occur at successively higher ranges of drive current as shown in FIG. 1.
The type I multimode operation tends to occur nearest the threshold of the semiconductor laser. However, it can extend to the semiconductor's upper current limit. As shown in FIG. 2, the type I multimode condition is characterized by a strong main mode and two weak symmetric sidemodes. The sidemode ratio may be anywhere from approximately -10 to -30 db. This is the mildest form of laser multimode operation with the linewidth of the main mode comparable to that of a single oscillating mode (approximately 50-100 kilohertz "KHz").
The type II multimode operation, as shown in FIG. 3, tends to occur at higher current levels than type I but it sometimes occurs right at the threshold current of the semiconductor. There may be several modes with approximately equal energy levels and there may be no discernable main mode. The linewidths of the individual modes may be somewhat broadened compared to type I and of approximately 100 KHz to 10 MHz. With the type II multimode condition, the laser energy may be spread over several frequencies and the laser system will no longer function in coherent applications.
The type III multimode operation occurs at the highest current range of the semiconductor. The energy is spread more or less uniformly over many frequencies, as shown in FIG. 4, each of which is broadened to a GHz or more. The type III multimode operation is the regime of "coherence collapse". This collapse is generally due to the onset of optical chaos in the external cavity where no single main mode is amplified.
Attempts have been made to control external cavity lasers to prevent multimode operation. M. Ohtsu et al describe one such attempt in the "Journal of Lightwave Technology", Vol. 7, No. 1, Jan. 1989, in the article titled "A Simple Interferometric Method for Monitoring Mode Hopping in Tunable External-Cavity Semiconductor Lasers". Their experimental apparatus is shown in FIG. 5.
In the Ohtsu et al experiment, an automatic control was built using a electro-optical servo loop to suppress multimoding. A fiber delay line 501 was used to increase the temporal overlap of the laser modes thereby increasing the resultant optical signal. A local oscillator 503 and mixer 505 were used to down convert the beat signal from the detector 507, typically a few GHZ, to a few hundred MHZ. The down conversion process produces a radio frequency (RF) signal that is used to control a piezo electric transducer (PZT) 509.
Ohtsu et al. uses the presence of the mode beat signal to change the cavity length. This is accomplished by varying the voltage to the PZT and thereby moving the grating 511. The movement of the grating does not change the pass bandwidth of the optical feedback, only the feedback phase. If the change in the grating position causes the laser output to become single mode, the beat signal disappears and the present voltage to the PZT is maintained thereby maintaining the feedback phase at the proper value.
The inventor's experimental results have shown that adjusting the length of the cavity does not suppress all three types of multimode operation. Although changing the length of the cavity suppresses type I multimode operation, it is only partially effective in suppressing type II multimode operation and it is ineffective in suppressing type III multimode operation. Additionally, the cavity length sometimes requires readjustment if the laser current is changed even within one particular regime of multimode operation for which it is effective.
In the "Journal of Lightwave Technology", Vol. LT-5, No. 4, April 1987, N.A. Olsson et al published an article titled "Performance Characteristics of 1.5 um External Cavity Semiconductor Lasers for Coherent Optical Communication". This article describes an external cavity laser system that uses feedback to control the cavity length with a PZT. Also described is an intra-cavity etalon that narrows the passband of the cavity. The angle of the etalon is changed to tune between the internal modes of the laser. As is well known in the art, the etalon will periodically narrow the optical passband width of the grating as the etalon is increasingly tilted in the intracavity beam. The Olsson et al system is shown in FIGS. 6 and 7.
The arrangement of the basic elements of the Olsson et al external cavity laser is best shown in FIG. 6. The light from a laser diode 601 is collimated by a lens 603 and projected through an etalon 605 and retroreflected back to the laser by a grating reflector 607. A PZT element 609 is mechanically coupled to the reflector 607 and is used to adjust the optical path length.
To stabilize the laser, a feedback system is provided as shown in FIG. 7. The feedback system uses a beat detector 701, a mixer 703 to convert the detector output to a lower frequency, a low pass filter 705 to convert the mixer output to a DC (direct current) control level and a high voltage amplifier 707 to amplify the DC control level to the high voltages required to drive the PZT (not shown).
Tuning of the Olsson et al external cavity laser is divided into coarse, medium, and fine tuning. Coarse tuning is performed by rotating the grating reflector 607 and selecting the internal mode which is closest to the desired wavelength. This provides a tuning range of approximately 900 A (Angstroms). The internal modes refer to the longitudinal modes of the laser diode without the external cavity. These modes appear because of the imperfect anti-reflection coating on the laser facet. Medium tuning is the tuning of the laser in between its internal modes, a range of appoximately 19 A. This tuning is achieved by manually adjusting the intra cavity etalon 605 in combination with a fine rotation of the grating reflector 607. Fine tuning is performed by adjustments to the external cavity length that varies the laser frequency within the megahertz range. The PZT 609 is connected to the reflector 607 to facilitate this adjustment.
While the Olsson et al external cavity laser system may eliminate all three types of multimode operations with multiple manual adjustments, there still exists the need for a laser system that eliminates all three types of multimode operations automatically.